This invention relates to a parallel-plane antenna known as a cheese antenna or pillbox antenna.
In accordance with a generally accepted definition, a parallel-plane antenna of the cheese or pillbox type is essentially constituted by two parallel plates limited by a cylindrical-parabolic end-wall illuminated by a source placed on its focal line as shown in FIG. 1. The source can be a simple monomode horn or a multimode source. Depending on the height H or distance between the two parallel plates, a number of operating modes can be contemplated such as the TEM, TE and TM modes. Particulars on this type of antenna are given in "Les Antennes" by L. Thourel, published by Dunod, page 285, 1976 edition, and in "Microwave Antenna Theory and Design", Vol. 12, page 459 of the "Radiation Laboratories Series" (1949 edition).
A disadvantage of this type of antenna, however, lies in the fact that its radiating aperture is occulted by the source, thus having the effect of impairing the radiation characteristics of the antenna.
One solution for eliminating this occulting effect without increasing the depth of the antenna consists in constructing a parallel-plane antenna of the cheese or pillbox type having a folded configuration as shown in FIG. 2 in which the feed is separate from the radiating portion. This solution provides a higher gain than the simple pillbox or cheese antenna.
This antenna is constituted by two pillbox or cheese antennas 41 and 43 coupled together by means of a slot 44 extending over the entire length of the parabolic end-wall which is common to the two antennas. The slot 44 is so dimensioned that the assembly formed by the two cheese elements 41 and 43 and the slot 44 constitutes an adapted folded waveguide.
However, one drawback attached to these two antenna designs arises from the difficulties presented by the mechanisms employed for driving the antenna in rotation. In point of fact, the choice of an antenna having a long focal distance makes is necessary to place the excitation source at a substantial distance from the center of gravity of the antenna. This has the effect of complicating the supporting structure in proportion to said distance and increasing the capacity of the mechanisms employed for driving the turret on which the antenna is mounted.